In recent years, audio cassette tapes have been required which provide a low noise level and well-balanced frequency characteristics which produce excellent output in all frequency bands. Since the recent music sources tend to be digitalized in compact disc or the like for ultrahigh fidelity or extremely low noise, for example, such requirements have grown ever-increasingly in audio cassette tape technology.
Similarly, video tapes have been required to provide a higher video output and a lower noise level.
To this end, it is conventional knowledge that if a ferromagnetic iron oxide powder is used as ferromagnetic powder to be incorporated in a magnetic recording medium, then a lower noise level can be obtained by using a ferromagnetic powder having a short average length in the long axis. However, this approach can reduce the noise level but is lacking in frequency characteristics desired.
In order to significantly improve the frequency characteristics of such a magnetic recording medium, a multi-layer magnetic tape comprising two or three magnetic layers has been proposed. In one example of such a multi-layer magnetic tape, a magnetic layer is designed such that higher coercive force is given, i.e., better high frequency characteristics can be obtained towards the upper layer to provide a high output over all frequency bands (see JP-A-57-154635 and 53-16604 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, this approach is disadvantageous in that since the magnetic layers are definitely dissimilar from each other, the output reproduced therefrom is not uniform over all frequency bands, i.e., shows gradual or sudden drop in its frequency characteristic curve. This problem can be alleviated by addition of more layers which provides for improved frequently response. However, in the light of actual manufacturing considerations, the more layers that are contained in the magnetic tape, then, the more times that coating operations are needed, which increases the production costs.
In another example of such a multi-layer magnetic tape, in addition to the above mentioned consideration of coercive force, a ferromagnetic powder having a short average length in the long axis is incorporated in the upper magnetic layer to lower the noise level while a ferromagnetic powder having a long average length in the long axis is incorporated in the lower magnetic layer to improve the low frequency characteristics as disclosed in JP-A-59-172142.
However, if a ferromagnetic powder having a long average length in the long axis is incorporated in the lower magnetic layer, the surface of the magnetic layer thus obtained exhibits a poorer smoothness than that of a magnetic layer comprising a ferromagnetic powder having a short average length in the long axis. Thus, an upper magnetic layer coated on such a lower magnetic layer is influenced by the poor smoothness of the lower magnetic surface and, the coated upper magnetic layer in turn exhibits a poor smoothness as it replicates the poor smoothness of the underlying layer. Accordingly, though giving some improvement in the frequency characteristics, this approach is disadvantageous in that a poor high frequency response is given in the case of audio tapes and a reduction in sensitivity and an increase in noise cause deterioration in S/N ratio (i.e., signal/noise ratio) in video tapes. In recent years, video tapes have been required to provide a high image quality and an improved tone quality. In order to improve video performance or raise output, higher packing ferromagnetic powders must be used. In order to improve S/N ratio, ferromagnetic powders having a fine particle size must be used. In audio tapes, since the tone can be improved by raising S/N ratio, it has been required to minimize the size of ferromagnetic powder.
However, if a finer ferromagnetic powder is used to provide a better video performance, substantial print through of audio signals occurs, resulting in a pronounced deterioration in tone. Therefore, the lower limit of the particle size of ferromagnetic powder is limited at presently used particle sizes.
The print through of audio signals can be minimized not only by increasing the particle size of the ferromagnetic powder to be used but also by increasing the coercive force of the ferromagnetic powder. However, if the coercive force of ferromagnetic powder is raised, the output at a low frequency range is reduced, resulting in undesirable tone.